JOEL - The term "ghost" has been used in a few scriptures refering to our spirits, where for instance a body has given up the ghost(Gen. 49:33, Acts 5:10) or at the Lord's death where He "yielded up the ghost"(Matt. 27:50), or His spirit.
Then there is the perverted deffinition of ghost, made famous during the dark ages and in today's movies and horror stories, which describes ghosts as hideous demons of an unseen world that haunt old houses and graveyards or appear in bodily form to frighten mortals. These include apparitions, goblins, spectors, spooks, sprites, faries and the like. These do not really exist except in the minds of those who claim to see them.
There are however both good and evil spirits that can and do appear and interact with mortals. There have been many reports of spirits of those who have passed on who have appeared to loved ones for various reasons. These are personal manifestations meant solely for the benefit of the recipient.
Brigham Young taught that the spirit world is right here on earth with us[Discourses of Brigham Young, ed. John A. Widtsoe, pp. 376-81. Salt Lake City, 1946.], perhaps making it easy for spirits of the deceased to appear to people from time to time as the Lord allows and according to one's faith.
They are not spirits that are angry and won't leave. The evil spirits who torment people and posess their bodies may be those who followed satan and never obtained a body.